#include <bits/stdc++.h>

using namespace std;
using ll = long long;

void solve() {
	int a, b, c, d;
	cin >> a >> b >> c >> d;
	if (c < a && d > b) {
		cout << "A" << endl;
	} else {
		cout << "B" << endl;
	}
}

int main() {
	ios::sync_with_stdio(false);
	cin.tie(nullptr);
	solve();
	return 0;
}